Ad hominem

...an attempt to link the validity of a premise to a characteristic or belief of the person advocating the premise. The ad hominem is a classic logical fallacy, but it is not always fallacious; in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue.

One of the common errors made during discussions and arguments at Pharyngula is to confuse ad hominem with insult. An ad hominem response to an argument might be: "You're wrong because you're an idiot." However saying: "You're wrong because of this, that and t'other fact, you idiot" is not ad hominem but is insulting.